1. Field of the Invention
The present invention pertains to a pressure support system and a method of providing pressure support to a patient, and, more particularly, to a pressure support system and method having enhanced noise reduction, as well as a pressure control valve assembly for use in a pressure support system.
2. Description of the Related Art
Pressure support systems that provide a flow of gas to an airway of a patient at an elevated pressure via a patient circuit to treat a medical disorder are well known. For example, it is known to use a continuous positive airway pressure (CPAP) device to supply a constant positive pressure to the airway of a patient to treat obstructive sleep apnea (OSA). It is also known to provide a positive pressure therapy in which the pressure of gas delivered to the patient varies with the patient's breathing cycle or varies with the patient's effort to increase the comfort to the patient. It is further known to provide a positive pressure therapy in which the pressure provided to the patient changes based on the detected conditions of the patient, such as whether the patient is snoring or experiencing an apnea, hypopnea or upper airway resistance.
Conventional pressure support devices typically include a pressure generator, for example, a blower, piston, or bellows, that creates a flow of breathing gas at a pressure greater than the ambient atmospheric pressure. A patient circuit delivers the elevated pressure breathing gas to the airway of the patient. Typically, the patient circuit includes a conduit, e.g., a single limb or lumen, having one end coupled to the pressure generator and a patient interface device coupled to the other end of the conduit. The patient interface connects the conduit with the airway of the patient so that the elevated pressure gas flow is delivered to the patient's airway. Examples of patient interface devices include a nasal mask, nasal and oral mask, full face mask, nasal cannula, oral mouthpiece, tracheal tube, endotracheal tube, or hood. A single limb patient circuit includes an exhalation port, also referred to as an exhalation vent, exhaust port, or exhaust vent, to allow expired gas from the patient to exhaust to atmosphere. Generally, the exhaust vent is located in the conduit near the patient interface or in the patient interface device itself.
Many of these pressure support devices are used at night, especially where the function of the pressure support system is to treat a breathing disorder that occurs during sleep, such a sleep apnea. For this reason, the pressure support system must be quiet so as not to arouse the user or the user's bed partner. One source of noise addressed by the present invention is the exhaust assembly downstream of the pressure generator, which exhausts gas from the patient circuit to atmosphere. This is typically done in order to maintain the correct pressure or flow in the patient circuit. Typically, this exhaust assembly and associated exhaust path are located within a housing that contains the pressure generator.
Conventional pressure support devices with this configuration attempt to minimize noise due to exhausting of gas through the pressure control valve assembly to atmosphere by providing a dedicated muffler in the exhaust path. However, such a muffler is disadvantageous in that it adds significant cost, size, and weight to the pressure support system.
An example of a conventional pressure support system 1 with this configuration is shown in FIG. 1. The conventional system includes a blower assembly 2 having a blower housing 3, a fan 4 contained within the blower housing, a motor 5 for driving fan 4, an inlet or intake 6, and an outlet 8. These features are collectively referred to as a pressure generator 7. Inlet 6 is coupled to a first conduit 10 that communicates the inlet of the blower assembly to atmosphere. Outlet 8 of blower assembly 2 is coupled to a second conduit 12 that communicates a flow of breathing gas having an elevated pressure relative to ambient atmosphere created by the blower assembly to a third conduit 13 for delivery to a patient 20. A single housing 15, generally identified by the dashed line in FIG. 1, houses the components of the pressure support system.
Third conduit 13 has a first end 14 coupled to an end of second conduit 12 opposite outlet 8. Third conduit 13 also has a second end 16, opposite first end 14, that is coupled to a patient interface 18, which can be secured to patient 20 in a manner known in the art. Third conduit 13 is typically a flexible conduit to allow the patient to move freely while using the pressure support system. All of the conduits and devices for delivering the flow of breathing gas from the blower assembly to the patient's airway define a patient circuit 19. In the embodiment illustrated in FIG. 1, patient circuit 19 includes second conduit 12, third conduit 13, and patient interface 18.
During operation, motor 5 drives fan 4, thereby creating, at intake 6, a negative pressure relative to the pressure of the fluid, e.g., air, in the ambient atmosphere. In response to this negative pressure, fluid in the ambient atmosphere is drawn through first conduit 10 and intake 6 into blower housing 3, wherein the operation of fan 4 increases the pressure of the fluid. The fluid pressurized by fan 4 is delivered from blower housing 3 at outlet 8 into second conduit 12. The pressurized fluid flows through third conduit 13 to patient interface 18 for receipt by patient 20. An exhaust port 22 is provided at second end 16 of third conduit 13 for exhausting gases, such as the exhaled gases from the patient, to ambient atmosphere. Exhaust port 22 can have a variety of configurations that are well known in the art, and can be provided in third conduit 13, as shown, or in patient interface 18.
As noted above, there are many instances where it is desirable to control the pressure, and, hence, the flow of fluid, delivered to the patient by the pressure support system. For example, it is known to provide a continuous positive airway pressure “CPAP” device with the ability to change the pressure or flow of fluid delivered to the patient so that a commonly designed device can be used to provide pressure support therapy to patients with different pressure support prescription levels. Typically, the patient's therapy pressure is determined in a sleep study and then the CPAP device prescribed and is set to output that prescription pressure at all time during its operation. An example of a CPAP device that operates in this manner is the REMstar® and Solo® family of devices manufactured and distributed by Respironics, Inc. of Pittsburgh, Pa.
Unlike a CPAP device, which outputs a constant pressure at all times during its operation, it is also known, as described above, to provide a pressure support therapy in which the pressure of breathing gas delivered to the patient varies during the course of treatment. For example, it is known to vary the pressure of breathing gas delivered to the patient in synchronization with the patient's breathing cycle so that a lower pressure is delivered to the patient during the expiratory phase of the breathing cycle than is delivered during the inspiratory phase, so that the patient is not breathing out against a relatively high pressure. This mode of pressure support is typically referred to as “bi-level” pressure support. Examples of pressure support devices that have the ability to operate in this bi-level mode of ventilation are the BiPAP® family of devices manufactured and distributed by Respironics, Inc.
It is also known to vary the pressure of breathing gas provided to the patient based on the detected conditions of the patient, such as whether the patient is snoring or experiencing an apnea, hypopnea, or upper airway resistance. This mode of pressure support is typically referred to as “auto” or “smart” pressure support because the pressure support device determines the optimum pressure to deliver to the patient. An example of a device that adjusts the pressure delivered to the patient based on the whether or not the patient is snoring is the Virtuoso® CPAP family of devices manufactured and distributed by Respironics, Inc. An example of a pressure support device that actively tests the patient's airway to determine whether obstruction, complete or partial, could occur and adjusts the pressure output to avoid this result is the Tranquility® Auto CPAP device also manufactured and distributed by Respironics, Inc.
Typically, these latter two conventional modes of pressure support, i.e., bi-level and “auto” or “smart” CPAP modes of pressure support, require some means for the pressure support system to detect the condition of the patient so that the pressure or flow provided to the patient can be controlled based on this condition. For example, a bi-level pressure support device typically includes a flow sensor 24 that detects the flow of fluid in patient circuit 19 and a pressure sensor 26 that detects the pressure at patient interface 18. A controller 28 receives the flow signal and the pressure signal from flow sensor 24 and pressure sensor 26, respectively, and uses this information to determine when the patient has transitioned from the inspiratory phase to the expiratory phase of the breathing cycle, or vice versa, to control the pressure accordingly. In the “auto” or “smart” CPAP mode of pressure support, these pressure and flow sensors, or other sensors, such as a microphone, are used to detect snoring, apneas, hypopneas, etc. In addition, it is known to have the less sophisticated CPAP devices monitor the pressure or flow of breathing gas delivered to the patient and adjust the pressure or flow in a feedback fashion to meet the desired prescription pressure level.
There are several techniques for controlling the pressure or flow of breathing gas to the patient in a pressure support device. As discussed above, one such method involves providing a pressure control valve assembly 30 in patient circuit 19 to exhaust a portion of the breathing gas in patient circuit 19 to atmosphere through an exhaust conduit 32, thereby decreasing the pressure and flow delivered to the patient. Controller 28 typically controls the operation of valve assembly 30 based on the detected conditions of the patient to control the pressure or flow of breathing gas provided to the patient. Due to the relatively large amount of noise associated with this exhaust flow, which is indicated by arrow 34 in FIG. 1, it is known to provide a muffler 36 in exhaust conduit 32. However, as noted above, the present inventors appreciated that such a muffler is undesirable in that it adds significant cost, size, and weight to the pressure support system.
As noted above, the pressure control technique shown in FIG. 1 uses a pressure control valve assembly 30 to selectively exhaust gas from the patient circuit to control the pressure of the gas in the circuit during the patient breathing cycle. One conventional pressure control valve assembly 30′ that accomplishes this function is taught in U.S. Pat. No. 5,694,923 to Hete et al. and is shown in FIG. 2. Pressure control valve assembly 30′ taught by Hete et al. uses two poppet type valves 40 and 42. Valve 40 controls exhaust of gas from patient circuit 19 to atmosphere and valve 42 restricts the flow of gas through patient circuit 19 provided by a pressure generator (not shown). This combination of valves is controlled by a controller 44 and actuators 46 to control the pressure or flow of fluid through the patient circuit delivered to the patient.
For example, when operating in the bi-level mode of ventilation, the inspiratory positive airway pressure (IPAP) is set by controlling one of more of the following: (1) the amount of flow restriction provided by valve 42, (2) the amount of exhaust to atmosphere provided by valve 40, and (3) the amount of flow provided by the pressure generator. During exhalation, the expiratory positive airway pressure (EPAP) is set by closing valve 42, either completely or partially, and operating valve 40 to control the amount of exhaust from patient circuit 19 to atmosphere. By completely blocking the flow from the pressure generator using valve 42 or by making sure the flow out of valve 40 is greater than the flow through valve 42, the patient does not exhale against any pressure provided by the pressure generator. The EPAP pressure in patient circuit 19 created by the patient's exhalation is regulated by controlling the amount of exhaust provided by valve 40.
This pressure control technique is advantageous in that it allows for a rapid transition between inspiration and expiration pressures so that the patient does not exhale against a pressure or flow provided by the pressure generator and immediately receives the IPAP pressure at the start of inspiration. In addition, the two valves can be controlled in tandem to minimize the amount of gas vented to atmosphere while still providing the proper pressure IPAP and EPAP levels. However, this pressure control technique is disadvantageous in that it requires two valve elements, which increases the cost and size of the pressure support system.
Another conventional pressure control valve assembly 30″ that exhausts gas to atmosphere, at least during a portion of the breathing cycle, to control the pressure in the patient circuit is taught in U.S. Pat. No. 5,598,838 to Servidio et al. and is shown in FIGS. 3A and 3B. Pressure control valve assembly 30″ taught by Servidio et al. uses a slideable hollow piston 50 that moves as shown in FIGS. 3A and 3B to control the pressure in patient circuit 19. An actuator 52 provides the necessary force to move piston 50. In a first position shown in FIG. 3A, piston 50 allows gas to flow from the pressure generator to the patient while blocking flow out of patient circuit 19 through exhaust ports 54. The IPAP level in patient circuit 19 is set by controlling one of more of the following: (1) the amount of flow restriction provided by piston 50 over inlet port 56 and (2) the amount of flow provided by the pressure generator. In a second position shown in FIG. 3B, piston 50 blocks the flow of gas from the pressure generator while exhausting gas from patient circuit 19 through exhaust ports 54. The EPAP level is set by controlling the amount of exhaust from patient circuit 19 to atmosphere through exhaust port 54, which is accomplished by controlling the position of piston 50.
Pressure control valve assembly 30″ is advantageous in that it uses a single valve element, namely piston 50, to control the IPAP and EPAP pressures in patient circuit 19. This configuration is also advantageous in that it can be used to measure the pressure in patient circuit 19 by determining the force that urges piston to move toward the position shown in FIG. 3B. This piston pressure control valve, however, requires moving a relatively large piston in order to control the pressure in the patient circuit. In addition, during expiration, actuator 52 must urge piston 50 to the right to balance against the pressure in patient circuit 19 that tend to urge the piston to the left. For these reasons, actuator 52 must be able to deliver a relatively large force to control the position of piston 50. In addition, because the pressure in the patient circuit affects the position of the piston, it is relatively difficult to control the position of the piston, and, hence the pressure in the patient circuit, with a high degree of accurately with a minimal amount of energy. This valve assembly also does not allow for gas to be delivered from the pressure generator into the patient circuit while simultaneously exhausting gas from the patient circuit, which is helpful to ensure that the pressure generator and the gas in the pressure generator does not become over heated by the continued operation of the pressure generator while the flow from the pressure generator is blocked.
The two above-described pressure control techniques involve restricting the flow of gas from the pressure generator into the patient circuit or exhausting gas from the patient circuit, or both in the case of the dual poppet valve system, to control the pressure or flow of breathing gas delivered to the patient. However, another technique exists for controlling the pressure or flow of breathing gas delivered to the patient. This third technique involves controlling only the motor speed of the pressure generator, such as the motor speed of a blower that is used to create a flow of gas, so that the pressure generator outputs the gas at the desired rate or pressure without additional pressure control valves.
A disadvantage with a pressure support system that uses motor speed to control the pressure or flow of the gas delivered to the patient is that it is difficult to rapidly transition from one pressure level to another. For example, during a bi-level mode of ventilation it is necessary to provide breathing gas to a patient at a relatively high level during the inspiratory phase of their breathing cycle and to provide the breathing gas at a relatively low level during the expiratory phase. This can require changing the pressure level of the breathing gas delivered to the patient a relatively large amount, such as 15 cm H2O, over a relatively short period of time, such as 0.5 seconds. Because of the relatively high rotational velocity or the relatively large mass of the blower, i.e., impeller, it is difficult to increase or decrease the motor speed by the required amount in the necessary short time period to achieve these pressure variations.
If the blower speed is not reduced fast enough during the transition from the inspiratory to the expiratory phase of the breathing cycle, for example, the patient begins exhaling against a pressure head, which is generally not conducive to patient comport. Changing the motor speed to control the pressure or flow delivered to the patient also subjects the pressure generator, specifically the blower, to relatively large mechanical and electrical stresses, which increase the wear on the blower. Increased wear may reduce the blower's operating life or require that the blower be manufactured to withstand such stresses, which increases the overall cost of the blower.